The present invention relates to an organic electroluminescence (EL) panel and a method of forming the same.
The present application claims priority from Japanese Patent Application No. 2004-191198, the disclosure of which is incorporated herein by reference.
An organic EL panel has a display area which is constituted by a single or an array of surface-emitting elements. The surface-emitting elements are made of organic EL devices formed on a substrate. To form organic EL devices, bottom electrodes of various structures are formed on the substrate. Then, a pattern of organic layers including an organic luminescent layer is formed, and top electrodes are formed thereon.
FIG. 1 shows a sectional structure of an organic EL device for constituting a typical conventional organic EL panel. The organic EL device 10 is formed on a substrate 11, and has the layered structure that organic layers 20 including an organic luminescent layer are held between a pair of electrodes. To be more specific, an insulating film 13 is formed around a bottom electrode 12 which is formed on the substrate 11. The area on the bottom electrode 12, defined by this insulating film 13, makes a luminescent area S. In this luminescent area S, the organic layers 20 are laminated on the bottom electrode 12. A top electrode 14 is formed thereon.
Here, the organic layers 20 show an example of three-layer structure including a hole transporting layer 21, a luminescent layer 22, and an electron transporting layer 23, with the bottom electrode 12 and the top electrode 14 as an anode and a cathode, respectively. Other structures may also be used, including: one from which either one or both of the hole transporting layer 21 and the electron transporting layer 23 are omitted; one in which at least one of the foregoing layers is made of a plurality of layers; one in which a hole injection layer is formed on the anode side of the hole transporting layer 21; and one in which an electron injection layer is formed on the cathode side of the electron transporting layer 23. As for the bottom electrode 12 and the top electrode 14, the anode and the cathode may be inverted with the foregoing structure in reverse order.
In such an organic EL device for constituting an organic EL panel, a voltage is applied to across the bottom electrode 12 and the top electrode 14. Holes and electrons are thereby injected and transported into the organic layers 20 from the anode side and the cathode side, respectively, so that they are recombined for luminescence. Consequently, the organic layers 20 to be held between the bottom electrode 12 and the top electrode 14 must be deposited in uniform thicknesses. If the organic layers 20 in the luminescent area S have a locally thinner point, a leak current can occur in that point for poor luminescence.
To achieve the uniform thicknesses of the organic layers 20, it is of importance to improve the degree of flatness of the underlayer, or the bottom electrode 12. In general, when a bottom emission method for outputting light from the side of the substrate 11 is employed, the bottom electrode 12 is made of a transparent conductive film such as ITO (Indium-Tin-Oxide). The film is typically formed by sputtering deposition or electron beam (EB) deposition. Nevertheless, these depositions yield surface roughness on the order of several to several tens of nanometers, in terms of the maximum height (Rmax) of the surface roughness defined in JIS B0601. These figures have a considerable impact since the organic layers 20 are laminated in a thickness of 100 to 200 nm or so.
In view of this, a conventional technique described in Japanese Patent Application Laid-Open No. Hei 9-245965 has been proposed. This publication describes that the surface of the ITO bottom electrode formed by sputtering deposition or electron beam deposition is polished to 5 nm or less in the maximum height (Rmax) of the surface roughness defined in JIS B0601.
According to this conventional example, the surface of the bottom electrode is polished by several tens of nanometers, whereby projections on the surface can be ground off. There is a problem, however, that extremely deep pits might still remain. In particular, if foreign particles and the like adhere to the surface of the bottom electrode under deposition and cause deposition defects (pinholes) pits can remain in the areas of the deposition defects no matter how the surface is polished.
In addition, pits and projections remaining in/on the polished surface of the bottom electrode tend to form sharp edges. Another problem might thus occur since the edges accumulate electric charges and facilitate leak currents to the top electrode.
Moreover, after the bottom electrode is polished at the surface, abrasives (abrasive particles) may sometimes reside on the surface of the bottom electrode. This can produce the problem that the residual abrasives preclude the organic layers laminated thereon from having sufficient thicknesses, thereby causing leak currents.
Now, in view of outputting the light from the substrate side, the set thickness of the bottom electrode must be controlled according to the luminescence color so that the spectrum of the output light peaks at a desired wavelength. More specifically, the luminescence occurring from the organic layers 20 includes light which is reflected at the interfaces of the respective layers a plurality of times before transmitted through the transparent conductive film (bottom electrode) for emission. The organic EL device itself thus exhibits the function of an optical interference filter. Concerning the thickness of the bottom electrode, the spectrum of the output light varies due to the reflection and interference phenomena between the light which is output after reflection at the interface between the organic layers and the bottom electrode and the light which is output after reflection at the interface between the bottom electrode and the substrate. Consequently, the thickness control on the bottom electrode is an important design factor in forming an organic EL panel.
Nevertheless, when the bottom electrode is simply polished at the surface as in the conventional technique, the thickness to be ground off by the polishing can vary with the surface roughness. This makes it difficult to control the final thickness of the bottom electrode to a set value. That is, suppose that the bottom electrode is initially deposited to a thickness of ta, and the surface is polished by a known thickness of tb. Then, the final thickness of the bottom electrode can be set at ta-tb, whereas the surface does not always have the required flatness in this state. In other words, it is substantially impossible to set the polishing thickness tb to the known value when surface flatness is pursued. This means the problem that the bottom electrode cannot be finished to the desired thickness.